O2 sensor heater circuit malfunction (bank 1 sensor 1). You probably have a burned out Bank 1 Sensor 1 O2 sensor. Also check the wiring at the sensor for damage.
Start with replacing the thermostat.
replace cat
change your o2 sensor
I've got a better question, why is this question in the Chevy venture forum ?
Start with looking for a vacuum leak.
you stick some plastic explosives on it and run
pull the intake and fix the secondary runners
P0121 Throttle/Pedal Position Sensor A Circuit Range/Performance IvryJade writes: Yes, but how do I fix it?
A loose gas cap will cause this. Check that first then have a local parts store such as Advance clear the code for you.
Trouble code P1111 means: IAT Sensor Circuit Intermittent Voltage High
Code 12 is a period It flashes when the codes start and end so you will see code 12 usually three times, then the codes you need to fix then code 12 three time and the codes you need to fix will repeat and so on.
Rebuild trans. Pay someone to do it too.